Being handed a formula and asked “is this ionic or covalent?” is one of the quickest marks in the chemical bond chapter, if you have a rule you trust. Most students can explain how each bond forms but freeze when asked to classify an unfamiliar compound. This guide gives you a short, reliable decision method, the reasoning behind it, and the handful of exceptions SPM likes to test.
The one-line rule
Here is the rule that answers most questions instantly:
- Metal + non-metal → ionic.
- Non-metal + non-metal → covalent.
That is because an ionic bond needs one atom to give electrons away and another to take them, and metals are the electron-givers while non-metals are the electron-takers. When two non-metals meet, neither will give electrons up, so they share instead, and sharing is covalent. So the whole prediction comes down to spotting metals and non-metals, which the Periodic Table shows at a glance.
Step one: read the Periodic Table
Metals occupy the left and centre of the table; non-metals sit on the right-hand side. Group 1 and Group 2 elements, sodium, potassium, magnesium, calcium, are metals. Group 17 (the halogens), Group 18, plus oxygen, nitrogen, carbon and hydrogen are non-metals. If you can place each element as a metal or a non-metal, you can classify the compound. Revise the layout on our the periodic table of elements chapter page.
The method, step by step
- Identify the elements in the formula.
- Classify each as a metal or a non-metal using the Periodic Table.
- Apply the rule: a metal with a non-metal is ionic; all non-metals is covalent.
- Check for a polyatomic-ion exception (see below).
- Confirm with a property if you can, ionic compounds have high melting points and conduct electricity when molten or in solution; simple covalent compounds do not.
Worked classifications
- NaCl, sodium (metal) + chlorine (non-metal) → ionic.
- MgO, magnesium (metal) + oxygen (non-metal) → ionic.
- KBr, potassium (metal) + bromine (non-metal) → ionic.
- CO₂, carbon (non-metal) + oxygen (non-metal) → covalent.
- H₂O, hydrogen (non-metal) + oxygen (non-metal) → covalent.
- CH₄, carbon and hydrogen, both non-metals → covalent.
Five seconds each, once the rule is automatic.
The deeper reason: electronegativity
If you want to understand why the rule works, think about electronegativity, how strongly an atom pulls bonding electrons towards itself. Metals have low electronegativity and non-metals have high electronegativity. When a low-electronegativity metal meets a high-electronegativity non-metal, the difference is large, the non-metal pulls the electron away completely, and you get ions, an ionic bond. When two non-metals with similar, high electronegativities meet, the difference is small, neither wins the tug-of-war, and the electrons stay shared, a covalent bond. You can revise the term at our electronegativity glossary entry. This is also why a large electronegativity difference points to ionic and a small one to covalent.
The exceptions SPM likes to test
Two situations break the simple rule, and examiners know it.
First, compounds of only non-metals that are still ionic because they contain a polyatomic ion. Ammonium chloride, NH₄Cl, is ionic even though nitrogen, hydrogen and chlorine are all non-metals, because it is built from the ammonium ion (NH₄⁺) and the chloride ion (Cl⁻). The same is true of ammonium nitrate. So if you see the ammonium group, think ionic.
Second, compounds of two non-metals that ionise in water but are covalent as pure substances. Hydrogen chloride, HCl, is a covalent molecule, two non-metals sharing a pair, even though it forms ions when dissolved in water to make hydrochloric acid. The bond in the pure gas is covalent; do not be fooled by its behaviour in solution.
A quick confirmation check
If you are ever unsure, the physical properties settle it. Ionic compounds have high melting and boiling points and conduct electricity when molten or dissolved; simple covalent compounds melt easily and do not conduct. You can see how these properties follow from the bonding on our chemical bond chapter page.
Predicting bond type is a small skill, but it underpins how you answer structure and property questions across Form 4 and Form 5. If you would like a teacher to test you on tricky classifications like NH₄Cl and HCl until they feel automatic, our online one-to-one lessons with our experienced SPM Chemistry teachers give that practice, from RM50 an hour with a paid one-hour trial.
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